1. Field of the Invention
The present invention relates to an image forming apparatus for forming images on a transfer material,and specifically relates to an image forming apparatus for forming superimposed images.
2. Related Background Art
The applicant of the present invention has proposed a number of color image forming apparatuses for obtaining a full color image by arranging a plurality of image holding devices (photo-sensitive drums or the like) in parallel.
Such apparatuses are described, for exmple, in Japanese Unexamined Patent Publication (Kokai) No. 58-23074 and Japanese Unexamined Patent Publication (Kokai) No. 58-95361 (corresponding to U.S. Pat. No. 4,591,903); Japanese Unexamined Patent Publication (Kokai) No. 58-95362, Japanese Unexmined Patent Publication (Kokai) No. 58-154856, Japanese Unexamined Patent Publication (Kokai) No. 58-207021, and Japanese Unexmined Patent Publication (Kokai) No. 59-31976 (corresponding to U.S. patent application No. 521,832 filed Aug. 10, 1983); and Japanese Unexamined Patent Publication (Kokai) No. 59-46659, Japanese Unexamined Patent Publication (Kokai) No. 59-50460, Japanese Unexamined Patent Publication (Kokai) No. 59-42879, all of which are incorporated herein by reference.
In an image forming apparatus of this type, an overlapping aberration (chromatic aberration) among respective colors at the time of multitransfer becomes an extremely large problem.
This problem will be explained in more detail with specific reference to FIGS. 12 to 17.
FIG. 12 schematically shows a 4-drum type full-color image forming apparatus. The apparatus has image forming stations 101C, 101M, 101Y and 101Bk for forming images of cyan, magenta, yellow and black colors, respectively. These image forming stations 101C, 101M, 101Y and 101Bk are respectively provided with photosensitive drums 102C, 102M, 102Y and 102Bk and optical scanning means 103C, 103M, 103Y and 103Bk, as well as developing units, cleaners and other parts. In operation, a transfer material or sheet S is conveyed in the direction of an arrow by a conveyor belt 112 through these image forming stations so that images of cyan, magenta, yellow and black colors are successively formed in an overlapping manner, whereby a full color image is formed on the transfer material S. The apparatus has a mark detector 111 disposed downstream from the image forming station 101Bk as viewed in the direction of movement of the transfer material, more specifically, at a point which is spaced from the center of the photosensitive drum 101Bk by a distance l.sub.4. As will be seen from FIG. 12, a constant interval (l.sub.1 =l.sub.2 =l.sub.3) is left between the photosensitive drums of the adjacent image forming stations.
The mark detector 111 is capable of detecting register marks which have been formed by the photosensitive drums 102C, 102M, 102Y and 102Bk of the successive image forming stations 101C, 101M, 101Y and 101Bk and transferred to the conveyor belt 112. These register marks are used as misregistration images indicative of any misregistration of images of the respective colors.
In this type of image forming apparatus having a plurality of image forming stations 101C, 101M, 101Y and 101Bk, images of the respective colors are successively formed on the same surface of the same transfer material S. Any deviation of the actual image transfer position from the designated position in each image forming station causes problems such as misregistration of the images of different colors or overlapping of the images of different colors, with the result that the quality of the reproduced image is seriously impaired due to degradation in the color or an unacceptably large misregistration.
Misregistration of color images takes place in various forms. For instance, misregistration takes place in the direction of conveyance of the transfer material indicated by an arrow A in FIG. 13(a). This misregistration is referred to as "top margin misregistration". Misregistration also takes place in the direction of scan of image indicated by an arrow B in FIG. 13(b). This type of misregistration will be referred to as "left margin misregistration". Misregistration can take place also in oblique direction, as shown in FIG. 13(c). This type of misregistration will be referred to as "oblique misregistration". FIG. 13(d) illustrates misregistration attributable to error in magnification. Thus, the misregistration of the type shown in FIG. 13(d) will be referred to as "magnification error misregistration". Usually, misregistration occurs in the form of combination of two or more of these four types of misregistration.
The top margin misregistration shown in FIG. 8(a) is mainly attributable to deviation in the time when image formation begins in the respective image forming stations 101C, 101M, 101Y and 101Bk. The left margin misregistration shown in FIG. 13(b) is usually caused by deviations in the time when writing of an image begins, i.e., deviations in the timing of start of each main scan of the image, in the respective image forming stations 101C, 101M, 101Y and 101Bk. Referring now to the oblique misregistration, this type of misregistration is chiefly attributed either to angular offset .theta..sub.1 see FIGS. 14(a) to 14(c)) in the mount of scanning optical systems and angular offset .theta..sub.2 (see FIGS. 15(a) to 15(c)) of the axes of the respective photosensitive drums 101C, 102M, 102Y and 102Bk. Finally, the magnification error misregistration shown in FIG. 8(d) is attributable to error .DELTA.L in the length of the optical path between the optical scanning system and the photosensitive drum 102C, 102M, 102Y or 102Bk in each image forming station, i.e., the difference in the length of the scanning line expressed by 2.times..delta.S, as shown in FIGS. 16 and 17.
Various measures have been taken in order to eliminate these four types of misregistration. For instance, electrical adjusting means are used to electrically adjust the scan timing by a light beam so as to eliminate top margin misregistration and left margin misregistration. On the other hand, for the purpose of eliminating oblique misregistration and magnification error misregistration, means are used for adjustably mounting the optical scanning units (referred to as "scanners" hereinafter) and the photosensitive drums 102C, 102M, 102Y and 102Bk so as to allow the positions and angles of these units to be adjusted to eliminate these misregistrations. Thus, the mounting positions and the mounting angles of the scanners and photosensitive drums directly affect the oblique misregistration and magnification error misregistration, so that these misregistrations can be eliminated by adjusting the mounting positions and angles of the scanners and drums, as well as positions and angles of reflection mirrors which are disposed in the optical paths.
The top margin misregistration and the left margin misregistration may take place as temporal changes during long use of the image forming apparatus, and such temporal changes can be corrected rather easily by electrical adjusting means. However, adjustment of mounting positions and/or angles of the scanners, drums and reflection mirrors, which are adjusted for the purpose of eliminating oblique misregistration and magnification error misregistration, is very difficult. This is because a highly delicate and minute adjustment required since accurate the adjustment must be accurate down to the order of pixel size which is as small as 62 micrometers. Misregistration in the respective image forming station is caused also by other indefinite factors. For instance, misregistration may be caused by unstable running characteristics of the conveyor belt 112, e.g., winding and offsetting, reproducibility of mounting of the photosensitive drum after dismounting, and so forth. In addition, when the image forming apparatus is a laser beam printer, the top margin and left margin tend to fluctuate due to characteristics peculiar to this type of printer. It is also to be pointed out that the positional relationship between the photosensitive member and the optical system in each image forming station, which has been initially set correctly after final set-up and adjustment before installation, may be lost due to strains of structural parts of the apparatus which are liable to occur when the apparatus is moved or transported to another location. Such a change in the positional relationship, even if it is very minute, undesirably causes misregistration of color images. Readjustment for restoring the correct positional relationship is very complicated and difficult to conduct.
In an image forming apparatus which is designed to form images with a much higher resolution than ordinary electrophotographic machines, e.g., a laser beam printer which is capable of forming dots at a very small pitch such as 16 dots/mm, the misregistration of color images is caused even by a very small expansion or contraction of the structural parts of the apparatus attributable to a change in the ambient air temperature, as well as by a temporal change.
In order to obviate these problems, it has been proposed to correct any misregistration of color images formed by the respective photosensitive drums 102C, 102M, 102Y and 102Bk, by enabling a highly precise detection of color image misregistrations through detection of register marks which are formed in the respective image forming stations and transferred simultaneously with the color images to the carrier or conveyor such as a transfer belt, intermediate transfer member, rolled paper sheet, cut sheet or the like, typically a conveyor belt 112 (see FIG. 7).
The apparatus having such a correcting function, however, still encounters the following problem, due to the location of the mark detector 111 at a position which is spaced in the downstream direction by the distance l.sub.4 from the axis of the most downstream photosensitive drum 102Bk. Namely, a considerably long time is required for the mark detector 111 to detect all the register marks corresponding to the photosensitive drums 102C, 102M, 102Y and 102Bk formed on the conveyor belt 112. For instance, representing the running speed of the conveyor belt 112 by P (mm/sec), a considerably long time (l.sub.1 +l.sub.2 +.sub.3 +l.sub.4)/P seconds is required for the register mark formed by the photosensitive drum 102C to reach the mark detector 111. If the correction of misregistration has to be executed for each of successive copies, the feed of copy paper has to be done with a time interval which is not shorter than (l.sub.1 +l.sub.2 +l.sub.3 +l.sub.4)/P seconds. Thus, starting of each copying operation is seriously degraded.
If the apparatus is designed such that correction of misregistration is effected upon each detection of a register mark, particularly when the design is such that the misregistration correction and image formation are executed simultaneously, a problem is encountered in that the color images of diffrent colors formed in the successive image forming stations are partially registered and partially misregistered, so that the hue of the final color image is changed with the result that the quality of the final color image is seriously degraded.
This problem is serious particularly in the case where the misregistration is the one which is caused by deviation of the optical scanning pitch, i.e., when the misregistration is the left margin misregistration. In this case, if the correction is executed during formation of the image, distinctive regular unevenness of image density is caused in a region which is to be reproduced in a uniform halftone, resulting in a critical defect in the final image.
If the detected misregistration is the oblique misregistration or the magnification error misregistration, correction of such misregistration requires geometrical rearrangement or relocation of the constituent parts. If such a relocation is executed during the image forming operation, there is a risk that the quality of the final color image is seriously impaired because the change in the hue and the regular unevenness are multiplied by each other as a result of vibration caused by the relocation.